Social network’s vast reach and micro-targeting tools are manna for political advertisers

Citigroup Inc. projects that spending on political ads on Facebook Inc. could surpass spending on Alphabet Inc.’s Google this year, reversing the historical pattern. This is no small accomplishment, considering how powerful search advertising remains, as a conduit for motivated donors and volunteers.

This reflects both Facebook’s vast reach and the tools it offers advertisers to target ever-narrower segments of its users. For campaigns striving to get supporters to the polls, as well as change minds, this ability to “micro-target”...